In sodium chloride electrolysis, hydrogen is evolved from an alkaline solution. Conventionally, the cathodes in the process are made of iron, copper, steel, or nickel. Nickel electrodes can be either solid nickel or nickel plated.
As mentioned in Offenlegungsschrift EP 298 055 A1, nickel electrodes can be coated with a metal from sub-group VIII, especially the platinum metals (inter alia Pt, Ru, Rh, Os, Ir, or Pd), of the periodic system of the elements or with an oxide of such a metal or with mixtures thereof. After a calcination process, the corresponding noble metal oxides are then usually present on the surface.
The electrode so produced can be used, for example, in sodium chloride electrolysis as the cathode for hydrogen development. Many coating variants are known, because the coating of metal oxides can be modified in very different ways so that different compositions form on the surface of the nickel electrode. According to U.S. Pat. No. 5,035,789, the cathode used is, for example, a ruthenium-oxide-based coating on nickel substrates.
Once in operation, the plating on the nickel electrode degrades and causes the cell voltage to increase, making necessary to re-coat the electrode. This is technically complex, because the electrolysis must be stopped and the electrodes must be removed from the electrolytic cells. An object of the invention is, therefore, to find a simpler method for increasing or restoring performance.
ELTECH has published and offered a technique with which a voltage reduction of from 200 to 300 mV as compared with untreated nickel electrodes can be achieved. In this technique, a noble-metal-containing solution of unnamed composition and constituents is applied in situ, i.e. during operation of the electrolysis, to the cathode side of the sodium chloride electrolysis in membrane cells. The solution is to be added during operation of the cell and is to lower the cell voltage.
According to the teaching of patent specification U.S. Pat. No. 4,555,317, iron compounds or finely divided iron is added to the catolyte in order to lower the cell voltage during sodium chloride electrolysis. The ELTECH publication contradicts this teaching, however, because, according to the information from ELTECH, coating the cathodes with iron is said to interfere with the electrolysis and to increase the cell voltage.
According to the further known Offenlegungsschrift EP 1 487 747 A1, a 0.1 to 10 wt. % platinum-containing compound is added to sodium chloride electrolysis. The solution of the platinum-containing compound is added to the water that forms the catolyte, from 0.1 to 2 liters of the aqueous solution of the platinum-compound-containing solution being added per liter of water.
According to JP 1011988 A, the activity of a deactivated cathode based on a Raney nickel structure with low hydrogen overvoltage is restored by adding, into the catolyte, a soluble compound of a metal of the platinum group to the sodium hydroxide solution during operation of the sodium chloride electrolysis. For example, a sodium chloride electrolytic cell with 32 wt. % sodium hydroxide solution, a salt concentration of 200 g/l of sodium chloride is operated at 90° C. and with a current density of 2.35 kA/m2. The cathode is subjected to currentless nickelling for pretreatment and then nickel-plated in a nickel bath. Platinum chlorate, for example, was metered into the catolyte as the active compound, which resulted in a reduction in the cell voltage by 100 mV.
According to U.S. Pat. No. 4,105,516, metal compounds which are to lower the hydrogen overvoltage and accordingly reduce the cell voltage are added to the catolyte during the electrolysis of alkali metal chlorides. The examples given in U.S. Pat. No. 4,105,516 in turn describe the metering and effects that arise by addition of an iron compound added to the catolyte of a sodium chloride diaphragm laboratory cell. The cell has an anode, consisting of expanded titanium metal, which is coated with ruthenium oxide and titanium oxide. The cathode consists of iron in the form of extended metal. The examples show the use of cobalt solution or iron solution at the iron cathode. Reference has already been made above to the disadvantages of iron compounds in the treatment of coated nickel electrodes.
According to the further known patent specification U.S. Pat. No. 4,555,317, it is known that sodium chloride electrolysis can be started with a nickel-coated copper cathode. An initial metering under electrolysis conditions of the cell was carried out with hexachloroplatinic acid in three steps. In the first step, 2 mg of platinum were metered in per 102 cm2, i.e. 0.02 mg/cm2, in the second step about 0.03 mg/cm2 and in the third step about 0.2 mg/cm2. The cell voltage was lowered by a total of about 157 mV.
According to U.S. Pat. No. 4,160,704, metal ions having a low hydrogen overvoltage can be added to catolytes of a membrane electrolytic cell for sodium chloride electrolysis in order to coat the cathode. The addition takes place during the electrolysis. However, the only example given is the addition of platinum oxide in order to improve an iron or copper cathode.
Sodium chloride electrolysis according to the membrane process is known in the prior art. The process is carried out as follows: a sodium-chloride-containing solution is fed to an anode chamber having an anode, and a sodium hydroxide solution is fed to a cathode chamber having a cathode. The two chambers are separated by an ion-exchange membrane. Joining multiple anode and cathode chambers forms an electrolyser. The product streams from the anode chamber include chlorine and a less concentrated sodium-chloride-containing solution. The product stream from the cathode chamber includes hydrogen, and a more highly concentrated sodium hydroxide solution than was fed thereto. The volume flow of sodium hydroxide solution fed to the cathode chamber is dependent on the current density and the cell design. At a current density of, for example, 4 kA/m2 and with the cell design of UHDE, Version BM 3.0, the volume flow of lye to the cathode chamber is, for example, between from 100 to 3001 l/h, with a concentration of the sodium hydroxide solution that comes off of from 30 to 33 wt. %. The geometrically projected cathode area is 2.71 m2, this corresponds to the membrane area. The cathode is made of specially coated extended nickel metal provided with a special coating (manufacturer e.g. DENORA) in order to lower the hydrogen overvoltage.
The cathode coatings in sodium chloride electrolysis conventionally consist of platinum metals, platinum metal oxides or mixtures thereof, such as, for example, a ruthenium/ruthenium oxide mixture. As is described in EP 129 374, the platinum metals that can be used include ruthenium, iridium, platinum, palladium and rhodium. The cathode coating does not have long-term stability, in particular not under conditions in which electrolysis does not occur or during interruptions in the electrolysis, during which pole reversal processes, for example, can occur. Accordingly, more or less pronounced damage occurs to the coating over the operating time of the electrolyser. Likewise, impurities which pass, for example, from the brine into the lye, such as, for example, iron ions, can become deposited on the cathode or especially on the active centres of the noble-metal-containing coating and as a result can deactivate the coating. The consequence is that the cell voltage rises, with the result that the energy consumption for the production of chlorine, hydrogen and sodium hydroxide solution increases and the economy of the process is markedly impaired.
It is likewise possible for only individual elements to exhibit damage to the cathode coating, and it is not always economical to stop the entire electrolyser therefor and remove the element with the damaged coating, because this is associated with considerable production losses and costs.
Methods for improving nickel electrodes for sodium chloride electrolysis which are coated with elements of the platinum metals (sub-group VIII of the periodic system), referred to hereinbelow as platinum metals, their oxides or mixtures thereof, have not hitherto been directly known from the prior art.